This invention relates generally to apparatus and methods for polarizing light and particularly to apparatus and methods for polarizing light that is propagating in an optical fiber. Still more particularly, this invention relates to a form birefringent fiber optic polarizer and methods of fabrication and use thereof.
A polarizer is a device that removes selected polarization components from a light wave. Some familiarity with propagation and polarization of light within an optical fiber will facilitate an understanding of both the present invention and the prior art. Therefore, a brief discussion of fiber optic waveguides, normal modes of propagation of light in such waveguides and polarization of light is presented.
It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. Any electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate unattenuated are the normal modes.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is said to be a transverse electric (TE) mode. If only the magnetic field vector is perpendicular to to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode. None of the normal modes require a definite direction of the field components; and in a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis.
The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for each mode. If all the electric field vectors in a wave point in only one particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude and 90.degree. out of phase, the electric field is circularly polarized because the net electric field is then a vector that rotates around the optic axis at an angular velocity equal to the frequency of the wave. If the two linear polarizations have unequal magnitudes and phases that are neither equal nor opposite, the wave has elliptical polarization. In general, any arbitary polarization can be represented by either the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two oppositely directed elliptical polarizations having orthogonal semi-major axes.
The velocity of an optical signal depends upon the index of refraction of the medium through which the light propagates. Certain materials have different refractive indices for different polarizations. A material that has two refractive indices is said to be birefringent. The polarization of the signal propagating along a single mode optical fiber is sometimes referred to as a mode. A standard single mode optical fiber may be regarded as a two mode fiber because it will propagate two waves of the same frequency and spatial distribution that have two different polarizations. Two different polarization components of the same normal mode can propagate through a birefringent material unchanged except for a difference in velocity of the two polarizations.
The amount of birefringence is used herein to mean the difference between the two refractive indices of a medium that guides a light wave. Controlling the amount of birefringence permits the control of the polarization of a light signal output from a length of fiber optic material. If the wave propagated by a fiber comprises two linear polarization components, increasing or decreasing the difference between the refractive indices of the fiber provides means for controlling the optical length of the fiber for each of the two polarizations. If the fiber is birefringent, then the two polarization components will be shifted in phase as they propagate along the fiber. Since the velocity of light in an optical fiber is v=c/n, where c is the free space velocity of light and n is the refractive index of the fiber, the polarization component having the lower refractive index will have a smaller transit time in the fiber than will the component having the higher refractive index. Many fiber optic systems have operational characteristics that are highly dependent upon the polarization of the light guided by the optical fiber. Such systens include optical gyroscopes and interferometric sensors. In order to obtain measurements of the desired accuracy, it is essential that the light have only a single polarization because only light waves of the same polarization produce the desired interference patterns.
Optical wave behavior in layers of two materials having different refractive indices optically and periodically stratified differs from that in a uniform medium. If the thickness of each layer is sufficiently small compared with the light wavelength and the number of layers is sufficiently large, the compound medium is birefringent. Form birefringence occurs in an ordered arrangement of layers of optically isotropic materials having dimensions large compared with the molecules of the materials, but small when compared with the optical wavelength propagating in the fiber.
Differential attenuation metal clad fiber optical polarizers based on the difference of approximately two orders of magnitude between the attenuation coefficients between the TE and TM modes have been described in the prior art. Such polarizers employ a relatively thick metal coating over a portion of the core of an optical fiber from which the cladding has been removed. As the waves impinges upon the portion of the cladding having the metallic coating, the distribution of electromagnetic fields changes so that one linear polarization is attenuated much more strongly than the other linear polarization.
Electric field components parallel to the metal coating cause ohmic heat in the metal and are rapidly attenuated. Electric field components parallel to the metal coating are attenuated only about 1% as strongly as the perpendicular electric fields. The prior art metal clad polarizers require a long interaction length in order to achieve a high extinction ratio. However, in order to obtain a high extinction ratio, these differential attenuation polarizers incur a high insertion loss. The extinction ratio of a polarizer is the measure of its efficacy in reducing the intensity of an undesired polarization relative to that of a desired polarization. The insertion loss is the ratio of the power of the desired polarization lost by transiting the polarizer relative to the initial power in the desired polarization. Even with a relatively long interaction length, polarizers based on differential attenuation give extinction ratios of about 24 dB when the insertion loss is restricted to an acceptable amount.
Since differential attenuation polarizers convert the energy in the undesired polarization into heat, the intensity of the wave having the undesired polarization cannot be monitored using a photodetector. Therefore, the differential attenuation polarizer is unsuitable for use with a polarization controller and a feedback system to provide optimum intensity of the desired polarization.
Previous fiber optic polarizers include the crystal polarizer disclosed in U.S. Pat. No. 4,386,822. A length of fiber optic material from which a portion of the cladding has been removed to form an interaction region is placed adjacent a birefringent crystal. The birefringent crystal is chosen such that it has a first index of refraction greater than or equal to that of the fiber core for the undesired polarization and a second index of refraction equal to or slightly less than that of the fiber cladding for the polarization that is desired to be propagated in the fiber. An exponentially decaying portion of the field guided by the fiber extends beyond the core boundary into the cladding. This decaying portion of the field is called the "evanescent field". The evanescent field of light guided by the fiber interacts with the birefringent crystal, and light of an undesired polarization couples to the birefringent medium and does not propagate in the fiber past the interaction region. Light of the desired polarization is unaffected by the birefringent crystal and is guided by the fiber.
Although the crystal polarizer is capable of providing the desired extinction ratios with low insertion loss, the operational characteristics of such polarizers are temperature dependent. The temperature dependence of such devices arises primarily from the temperature dependence of the refractive indices of the crystal. If the second refractive index of the crystal changes with temperature to exceed the refractive index of the cladding, then the crystal device ceases to function as a polarizer. If the refractive index of the crystal becomes appreciably less than that of the cladding, then some of the undesired polarization will be reflected at the crystal-fiber interface and will thus remain in the fiber rather than coupling into the crystal. A fiber optic gyroscope requires a polarizer with an extinction ratio greater than 100 dB. A crystal polarizer set to provide an extinction ratio of 100 dB at 24.degree. C. may have an extinction ratio of only 24 to 30 dB if the temperature increases to 30.degree. C.
Form birefringent polarizers using evanescent field coupling to form a bulk wave from the undesired polarization are known in the art. Evanescent field form birefringent polarizers require precise index matching of the core and cladding refractive indices. Such precise control of the refractive indices is difficult to achieve.